Monday, February 16, 2009

Evolution in a Nutshell

Sorry I haven't been writing much lately.  My personal life has been evolving.  More on that later.  But I would be remiss if I didn't have some kind of post for Charles Darwin's 200th birthday, which was on February 12th, 2009.  To celebrate, let me give you evolution in a nutshell.  I want to take a slightly different tack than most other treatments.  I want to argue that Darwin's central brilliant idea is so close to a tautology that it can't be wrong.

There are two things which determine whether a creature reproduces: the traits it was born with and the environment it lives in.  This is true even if you believe (which I don't) that a deity has an active hand in forming the creature, or in affecting its environment.  Now, given a bunch of creatures in that same environment, some will have inborn traits that make it more likely they will survive and reproduce, and some will have traits which make it less likely they will survive and reproduce.  For example, if there are a bunch of feral cats in Minnesota, those with heavy fur are more likely to survive than those with very light fur.  But if they are in Mexico, those with light fur are more likely to survive (all other things being equal).

Well, Darwin's central idea is Natural Selection by Survival of the Fittest.  Those creatures which are most suited to the given environment are the ones which are most likely survive.  It is almost tautological, because what else can "fittest" mean than "most likely to survive?"   And what does the "selection" entail other than "having survived"?  Nothing.  Darwin simply pointed out the obvious.  If you have a population of creatures, the ones best suited for the environment are the ones most likely to survive and reproduce.   And how can you tell which creature are the ones best suited for the environment?  Because they are the ones which survived!

So, then,  what's the big deal if it has to be true, almost by definition, that the "fittest" are "selected"?    The answer is that this simple mechanism, which as we have shown has to be true almost by definition, is enough to explain the evolution of life.  How did drug-resistant bacteria arise?  There were some bacteria in the huge population of bacteria which were resistant, and they took over in the patient because they were the fittest (they survived the drugs).  How did polar bears get such thick fur? Because their ancestors who had thinner fur did not survive as well.

Some people are fine with the above until one gets to major changes in species or complicated organs like a wing.  It would take me too long to delve into all possible considerations here, but the central answer is TIME.  Creatures have been roaming the Earth for a very long time, so even very slow change can have a dramatic effect.  Suppose each generation of creatures changes in some attribute by just 0.01%.  Then after 10,000 generations, they could have changed that attribute completely.  Even to take the human value for a generation, 20 years, that would take only 200,000 years.  That may sound  like a lot of time, but life has been on the planet for almost 4 billion years, which is 20,000 times 200,000 years.

So in summary, there has been a selection process going on for millions of generations of life, picking out creatures who are fittest for their environment. That process can explain the diversity of life we see now and in the past, and that is why, despite being essentially a tautology, Darwin's theory of evolution is the cornerstone of biology.  

Happy Birthday Darwin!

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